Step 1

Cut 1 medium onion in half and smash 4 garlic cloves. Place 3–4 lb. boneless beef short ribs, cut into 4" pieces, in a large heavy pot. Add 1 bay leaf, smashed garlic, an onion half, and 1 Tbsp. Diamond Crystal or 1¾ tsp. Morton kosher salt; pour in water just to cover meat. Bring to a boil; reduce heat and simmer, occasionally skimming foam from surface, until meat is nearly fork-tender, about 3 hours. Transfer meat to a large bowl. Strain braising liquid through a fine-mesh sieve into a medium bowl; discard solids. Set liquid aside.

Step 2

Meanwhile, cook 1 lb. red potatoes, peeled, cut into 1" pieces, in a medium pot of salted boiling water until just fork-tender, 8–10 minutes. Drain; set aside.

Step 3

When short ribs are almost done braising, coarsely chop remaining 1½ medium onions and finely chop remaining 4 garlic cloves. Heat 3 Tbsp. extra-virgin olive oil in another large heavy pot. Cook onions, garlic, 2 lb. tomatoes, coarsely chopped, 6 oz. tomatillos (about 3 medium), coarsely chopped, 1 red bell pepper, ribs and seeds removed, coarsely chopped, 1 green bell pepper, ribs and seeds removed, coarsely chopped, 1 small jalapeño, finely chopped, and 2 Tbsp. Diamond Crystal or 1 Tbsp. plus ½ tsp. Morton kosher salt, stirring occasionally, until vegetables are softened, 18–22 minutes. Add 2 Tbsp. tomato paste, 1½ tsp. ground cumin, and 1 tsp. ground coriander; season generously with freshly ground pepper. Cook, stirring occasionally, until vegetables are coated and mixture is pasty, about 4 minutes.

Step 4

Add short ribs, potatoes, and 3 cups reserved braising liquid to pot. Cook, stirring occasionally, until meat falls apart easily, 65–75 minutes. Let cool slightly; ladle into bowls and top with cilantro sprigs.